The MIRAGE-Shanghai experiment was designed to characterize the factors
controlling regional air pollution near a Chinese megacity (Shanghai) and was
conducted during September 2009. This paper provides information on the
measurements conducted for this study. In order to have some deep analysis of
the measurements, a regional chemical/dynamical model (version 3 of Weather
Research and Forecasting Chemical model – WRF-Chemv3) is applied for this
study. The model results are intensively compared with the measurements to
evaluate the model capability for calculating air pollutants in the Shanghai
region, especially the chemical species related to ozone formation. The
results show that the model is able to calculate the general distributions
(the level and the variability) of air pollutants in the Shanghai region, and
the differences between the model calculation and the measurement are mostly
smaller than 30%, except the calculations of HONO (nitrous acid) at PD
(Pudong) and CO (carbon monoxide) at DT (Dongtan).
<br><br>
The main scientific focus is the study of ozone chemical formation not only
in the urban area, but also on a regional scale of the surrounding area of
Shanghai. The results show that during the experiment period, the ozone
photochemical formation was strongly under the VOC (volatile organic
compound)-limited condition in the urban area of Shanghai. Moreover, the
VOC-limited condition occurred not only in the city, but also in the larger
regional area. There was a continuous enhancement of ozone concentrations in
the downwind of the megacity of Shanghai, resulting in a significant
enhancement of ozone concentrations in a very large regional area in the
surrounding region of Shanghai. The sensitivity study of the model suggests
that there is a threshold value for switching from VOC-limited condition to
NO<sub>x</sub> (nitric oxide and nitrogen dioxide)-limited condition. The
threshold value is strongly dependent on the emission ratio of NO<sub>x</sub> / VOCs.
When the ratio is about 0.4, the Shanghai region is under a
strong VOC-limited condition over the regional scale. In contrast, when the
ratio is reduced to about 0.1, the Shanghai region is under a strong
NO<sub>x</sub>-limited condition. The estimated threshold value (on the
regional scale) for switching from VOC-limited to NO<sub>x</sub>-limited
condition ranges from 0.1 to 0.2. This result has important implications for
ozone production in this region and will facilitate the development of
effective O<sub>3</sub> control strategies in the Shanghai region.